my betta i about 6 month old. for about a week now i have noticed him having trouble swimming? its almost like he is trying to to float on his side. and he is usually a good eater, he hasnt eaten in a week either.

sorry i didnt give enough info. he is by himself in a 1.5 gal tank. i change out his water and clean his gravel & house monthly. he eats the aqua care pellets. i dont do anything to his water other that the chlorine remover when i change his water. there is no apparent fungus or growths in him. although it is almost as if he can't see well anymore. i notice him hitting the sides of the tank. he doesnt flare at all either.

There is your problem. You should clean his tank weekly. Get a gravel vac and remove 25% of the water 2-3 times per week until he seems healthier then you can do it once a week. A more experienced member can probably help you out more. Good luck.